1. Field Of The Invention
The present invention relates to a planetary speed-up and -down gearing of the internally meshing type. It is to be noted that since a speed-down gearing also serves to increase a speed if the input side and the output side are reversed, the present invention is concerned with a speed-up and -down gearing inclusive of such a speed-up gearing, but the present invention will be explained in connection with only a speed-down gearing in the following description.
2. Description Of Related Art
There have been proposed various speed-down gearings using planetary gear mechanisms of the internally meshing type. Of the proposed speed-down gearings, a well-known planetary one includes an internal gear which has the arch-shaped tooth profile defined by a pin or a combination of a pin and a roller, and an external gear has a trochoidal tooth profile defined by epitrochoidal parallel curves. An inner pin or both the inner pin and an inner roller are loosely fitted with the external gear. With the external gear kept internally meshed with the internal gear, the external gear is swingingly rotated by an eccentric fitted in the external gear, so that input rotation is outputted while being reduced in speed. The above well-known system is used in a variety of speed-down mechanisms because it can transmit large torque and exhibit a large speed reduction ratio.
One example of the known system will be described below with reference to FIGS. 12 to 14.
FIG. 12 is a sectional view showing one example of the known planetary speed-down gearing and FIG. 13 is a sectional view taken along line B--B in FIG. 12.
An input rotary shaft 1 is inserted in and coupled to a hollow eccentric shaft 2 on which eccentrics 3.sub.1, 3.sub.2 are provided. The eccentric shaft 2 has a key way 4 formed in its hollow portion with a key 4A kept inserted in the key way 4. External gears 5.sub.1, 5.sub.2 are respectively fitted over the eccentrics 3.sub.1, 3.sub.2 through rollers 6. The external gears 5.sub.1, 5.sub.2 have outer teeth 7 formed along its outer circumference and having the trochoidal tooth profile. An internal gear 8 doubles as an outer casing and is fixedly provided. The internal gear 8 has the arc-shaped tooth profile defined by outer pins 9 which are internally meshed with the external gears 5.sub.1, 5.sub.2. As an alternative structure, if necessary, outer rollers may be loosely fitted over the outer circumferences of the outer pins 9. Inner pin holes 10 are formed in the external gears 5.sub.1, 5.sub.2 and inner pins 11 are loosely fitted in the respective inner pin holes 10. Inner rollers 11A are loosely fitted over the respective outer circumferences of the inner pins 11, and the inner pins 11 are tightly fitted to an inner pin holding flange 12. In the above structure, the inner rollers 11A may be dispensed with. The inner pin holding flange 12 is formed integrally with an output shaft 13.
The internal gear 8 is held by a pair of covers 14, 15 from both the sides in a sandwiched manner and fixed in place by bolts 16. Between the output shaft 13 and one cover 15, there are two bearings 17 provided at axially spaced positions.
It is to be noted that although the internal gear 8 is fixed and an output is taken out from the output shaft 13 in the above-mentioned known example, the structure may be modified, on the contrary, in such a manner as to fix the output shaft 13 and take out the rotation of the internal gear 8 as an output.
In the foregoing known example, one rotation of the input rotary shaft 1 corresponds to one rotation of the eccentrics 3.sub.1, 3.sub.2. However, the external gears 5.sub.1, 5.sub.2 are prevented from rotating about their axes by the inner pin holes 10 and the inner pins 11 so that they rotate swingingly. Assuming that the difference between the number of teeth of the external gears 5.sub.1, 5.sub.2 and the number of the outer pins (teeth) 9 is one, therefore, the outer teeth 7 of the external gears 5.sub.1, 5.sub.2 and the outer pins 9 as the inner teeth of the internal gear 8 are displaced (or slipped) in their meshing by one tooth for each rotation of the input rotary shaft 1. Accordingly, one rotation of the input rotary shaft 1 is reduced to 1/(the number of teeth) rotation of the external gears 5.sub.1, 5.sub.2, while the former's rotation is transmitted to the output shaft 13 through the inner pins 11.
FIG. 14 is a sectional view of the known system as disclosed in Japanese Patent Publication Sho 58-42382 and Japanese Patent Publication Sho 57-36455. In this known Cyclo Speed Reducer, the number of outer teeth of external gears 5.sub.1, 5.sub.2 is less the plural than the number of outer pins 9 of an internal gear 8 (the difference in the number of teeth being two in the illustrated example).
In the planetary speed-down gearings explained above, there exists a relative slippage in meshing between the outer teeth 7 of the external gears 5.sub.1, 5.sub.2 and the outer pins 9 of the internal gear 8. In practice, because of the outer pins 9 being rotatable about their axes, the internal gear 8 bears such a slippage at both the contact portions for supporting the outer pins and meshing with the outer teeth 7. Anyway, the outer teeth 7 and the outer pins 9 are subjected to a slippage in meshing therebetween. The outer teeth 7 and the outer pins 9 have surface roughness due to limitations in machining accuracy, as a result of which there inevitably give rise to noises at the slipping contact portions.
Furthermore, in the prior known planetary speed-down gearings explained above, since the diameter of each outer pin is made different dependent on the respective speed reduction ratios, parts of the internal gear cannot be shared by different types of gearings. Accordingly, the cost for quality control is increased, erroneous operations are unavoidable in assembly, and a large number of parts to be machined are required, thus resulting in the increased production cost.